Gardening Practices for Contaminated Urban Soils.

Urban soils have become widely contaminated by aerosol emissions of both trace elements and xenobiotics from the general urban atmosphere. The more volatile elements (e.g., Zn, Cd, Pb) have become strongly enriched near point sources or in center cities. Organics from automotive and stack emissions included polycyclic aromatic hydrocarbons and PCBs, but also historic pesticides such as DDT. Historic orchard pesticides (Pb, As, Hg, DDT) are found in suburban gardens converted from historic orchards. And pre-regulatory biosolids and composts may have carried substantial amounts of trace elements and xenobiotics into garden soils.

            Survey of urban garden soils has illustrated the center city effect on Pb, Zn and Cd, and the industrial source effects (incinerators, smelters). But surveys also show that houseside soils, or near painted surfaces have often become highly contaminated in Pb, up to 40,000 mg Pb kg^-1 in soil adjacent to a rural painted wood exterior home. Although patterns of traffic emitted Pb dispersal are evident in survey data, it is also evident that soil Pb is enriched adjacent to masonry walls because both roofs and non-painted walls serve to collect aerosol particles by surface tension, and these particles are then washed onto soils. On the other hand, humans bring in soil amendments which dilute the historic contaminants and may replace soils to improve fertility/productivity or remove contaminants.

            Garden contaminants of higher concern include Pb, Cd and PAH, and soil properties of importance in soil risk include pH, phosphate, and organic matter levels. Although soil Pb may show high contamination, the chemical form in the soil may have been changed over time to forms such as chloro-pyromorphite which has low bioavailability to animals which ingest soil. Because crops with adequate phosphate accumulate only low levels of Pb, the inadvertent ingestion of soil by hand-to-mouth play is usually the predominant route for soil Pb risk to young children, the group most sensitive to excessive soil Pb. Further, phosphate and phosphate-rich organic matter amendments (manures, composts, biosolids) can strongly reduce the bioavailability of soil Pb to mammals which ingest soil.

            Because human activities have caused variable and possibly important contamination of urban soils, it is wise to obtain soil analysis for major urban soil contaminants and soil fertility variables before adopting a garden. Some University Agricultural Extension Services offer soil fertility and Pb testing, and after evaluation of total vs. Mehlich-3 extractable Pb, Zn, Cd, etc., it is now apparent that an inexpensive extraction and multi-element analysis by ICP-AES can provide the information needed for safer gardening.

            When soil Pb exceeds recommended levels, one may seek analysis of bioaccessible soil Pb (an extraction shown to correlate with bioavailability of Pb in ingested soils to mammals). Further, by omission of a few common garden crops, one can prevent nearly all of the potential increase in dietary Pb due to urban soil Pb contamination. Of all garden crops, the low growing leafy vegetables and herbs, and the expanded hypocotyl root vegetables (carrot, beet, turnip, radish) can accumulate higher levels of soil contaminants than other garden fruits and seeds. Low growing leafy crops become enriched in fine soil particles by soil splash more than plant uptake. And the listed root crops accumulate Pb in the xylem tissues which grow thru the storage root.

            Most gardens are not so contaminated that gardeners need to be worried about safety of their crops or fear carrying soil into their homes. But soils near painted walls or porches, or heavily trafficked highways may be high enough that alternative soils should be developed such as raised beds. In fact, growing the leafy vegetables and herbs, and the listed root crops in clean soils in raised beds prevents nearly all potential risk from contaminated urban soils. And the root vegetables such as carrot which accumulate PAH and other xenobiotics in peel layers would remain at low levels of these compounds. None of other garden crops accumulate important amounts of xenobiotics in edible crop tissues.

            Because of the health, food security and economic benefits of urban gardening, it is important that we do not overestimate the potential for risk from contaminants in urban gardens. Gardeners can readily learn safe gardening practices and work to keep soil out of their homes. Extension and Master Gardener programs need to provide information to assist urban gardeners deal with contaminated soils including where they can obtain needed soil analyses for both fertility and contaminants.